1. Field of the Invention
The present invention relates generally to a handoff method in a mobile communication system, and in particular, to a handoff method in a mobile communication system supporting a concurrent service.
2. Description of the Related Art
In general, a CDMA (Code Division Multiple Access) mobile communication system cannot concurrently provide two difference services in an active state. That is, an existing mobile communication system provides only one of a voice service and a packet service, and rejects a newly requested service, which is different from the currently provided service.
FIG. 1 illustrates the structure of a common mobile communication system. More specifically, FIG. 1 illustrates a reference model of a 3G IOS (3rd Generation Interoperability Specifications) for a digital air interface between a mobile switching center (MSC) and a base station (BS), and between one base station and another base station.
Referring to FIG. 1, the mobile communication system includes a mobile switching center (MSC) 10, base stations (BS) 30 and 40, and a packet data service node or network (PDSN) 60. An interworking function (IWF) block 20 is connected to the mobile switching center 10, and a packet control function (PCF) block 50 is interconnected between the base station 30 and the PDSN 60. The mobile switching center 10 includes a call control and mobility management block 12 and a switch function block 14. The base station 30 is a source base station and the base station 40 is a target base station. The base stations 30 and 40 include base station controllers (BSCs) 32 and 42, and base station transceiver subsystem (BTSs) 34 and 44, respectively. The base station controller 32 includes an SDU (Service Data Unit) function block. The base station transceiver subsystems 34 and 44 are wirelessly connected to the mobile stations (not shown). Assuming that the mobile stations are connected to the mobile switching center 10 via the base station 30 to be provided with the voice service and connected to the PDSN 60 via the base station 30 to be provided with the packet service, a handoff occurs even when the mobile station moves to a service area of the base station 40 adjacent to the base station 30, in order to maintain the currently provided service. That is, during a handoff, the base station 30 serves as a source base station and the base station 40 serves as a target base station.
Signaling data between the mobile switching center 10 and the base station controller 32 is defined as an A1 interface, and user data (or traffic) therebetween is defined as A2/A5 (circuit data only) interfaces. An A3 interface is for a soft/softer handoff between the base stations, and is used for an exchange of signaling data and user traffic between frame selection functions of the target base station 40 and the source base station 30. An A7 interface is also for the soft/softer handoff between the base stations, and is used for an exchange of the signaling data between the target base station 40 and the source base station 30. A8/A9 interfaces are used for exchanges of user traffic and signaling data between the base station 30 and the PCF block 50, respectively. A10 and A11 interfaces are used for exchanges of user traffic and signaling data between the PCF block 50 and the PDSN 60, respectively.
In the CDMA system of FIG. 1, wired communication links between the base stations 30 and 40, and between the base station 30 and the mobile switching center 10 include a forward link for transmitting signals from the mobile switching center 10 to the base station 30, a reverse link for transmitting signals from the base station 30 to the mobile switching center 10, and a link between the base station 30 and the base station 40. A mobile station (not shown) interacting or connected to the base station transceiver subsystem 34 in the source base station 30 is connected to the mobile switching center 10 via the source base station 30 to be provided with the voice service, and connected to the PDSN 60 via the source base station 30 to be provided with the packet service.
It is assumed herein that the mobile station connected to the source base station 30 is provided with any one of the voice service and the packet service. A handoff process is performed when the mobile station moves from a service area of the source base station 30 to a service area of the target base station 40. Conventionally, the source base station 30 and the target base station 40 perform a handoff in accordance with the flows shown in FIGS. 2 and 3.
FIG. 2 illustrates a handoff process flow in the conventional CDMA communication system.
Referring to FIG. 2, when signal strength reported from a mobile station MS exceeds predefined signal strength, a source base station recommends one or more cells belonging to a target base station to perform a handoff by sending a handoff required message together with a list of the corresponding cells to the mobile station center MSC, in step 20a. At this moment, a handoff bit in the handoff required message is set to ‘1’ indicating the handoff. Further, the source base station activates a timer T7.
In step 20b, since the handoff bit in the handoff required message received from the source base station is set to ‘1’ indicating the handoff, the mobile switching center sends a handoff request message to a target base station. The handoff request message includes TIA/EIA-95 channel identity element. In particular, in the case of a handoff for an asynchronous data or facsimile service, the mobile switching center sends the handoff request message with a CIC (Circuit Identity Code) extension information element indicating a CIC code between SDU (Selection and Distribution Unit) and IWF (InterWorking Function).
In step 20c, upon receipt of the handoff request message from the mobile switching center, the target base station assigns a proper radio resource specified in the handoff request message and connects a call. In addition, the target base station sends a null forward traffic channel frame to the mobile station.
In step 20d, the target base station sends a handoff request acknowledge message to the mobile switching center. Further, the target base station activates a timer T9. The timer T9 remains activated until the mobile station is connected to the assigned radio channel.
In step 20e, in reply to the handoff request acknowledge message from the target base station, the mobile switching center prepares to switch the mobile station from the source base station to the target base station, and sends a handoff command message to the source base station. In response to the handoff command message, the source base station inactivates the timer T7.
In step 20f, in response to the handoff command message, the source base station sends a handoff direction message to the mobile station. For the handoff direction message, any one of general handoff direction message, extended handoff direction message and universal handoff direction message can be used. In addition, the source base station activates a timer T8. The source base station also activates a timer Twaitho, if the mobile station is permitted to return to the source base station.
In step 20g, the mobile station sends an MS Ack order message to the source base station in acknowledgement of the handoff direction message. At this point, the source base station inactivates the timer T8. If the handoff direction message is repeatedly sent at a high rate, the source base station may not require acknowledgement from the mobile station. In this case, the source base station does not activate the timer T8 in step 20f. 
In step 20h, the source base station sends a handoff commenced message to the mobile switching center in order to inform that the mobile station is ready to move to the target base station channel. In addition, the source base station activates the timer T306 until a clear command message is received from the mobile switching center. If the timer Twaitho has been activated, the source base station waits to inactivate the timer Twaitho before transmission of the handoff commenced message.
In step 20i, the mobile station sends a reverse traffic channel frame or a traffic channel preamble to the target base station.
In step 20j, the mobile station sends a handoff completion message to the target base station.
In step 20k, the target base station wirelessly sends a BS Ack order message to the mobile station.
In step 20l, the target base station sends to the mobile switching center a handoff complete message indicating a successful handoff of the mobile station. In addition, the target base station inactivates the timer T9.
In step 20m, the mobile switching center sends a clear command message to the source base station. Further, the mobile switching center activates a timer T315. In particular, in the case of a handoff for an asynchronous data or facsimile service, the mobile switching center sends a clear command message for clearing every resource including A5 connection in the previous base station. In reply to the clear command message, the source base station inactivates the timer T306.
In step 20n, the source base station sends to the mobile switching center a clear complete message indicating a successful clear. In response to the clear complete message, the mobile switching center inactivates the timer T315.
FIG. 3 illustrates a procedure for creating and transmitting a handoff required message in the source base station during a handoff according to the prior art.
Referring to FIG. 3, in step 10, the source base station receives a pilot signal strength measurement message (PSMM) from the mobile station MS and determines whether to perform a handoff, depending on the received PSMM message. If determined to perform a handoff, the source base station creates a handoff required message in step 11. The created handoff required message includes a single service option and an IS-2000 Service Configuration Record (SCR) related to this service. For example, the handoff required message includes an IS-2000 Service Configuration Record related to the voice service, or includes an IS-2000 Service Configuration Record related to the packet service. In step 12, the source base station sends the created handoff required message to the mobile switching center (MSC).
FIG. 4 illustrates a procedure for receiving in the target base station a handoff request message from the mobile switching center and processing the received handoff request message during a handoff according to a prior art.
Referring to FIG. 4, in step 21, the target base station checks the service option and the IS-2000 Service Configuration Record, specified in the handoff request message received from the mobile switching center MSC. In step 22, the target base station determines whether it is possible to accept a handoff. If it is possible to accept a handoff in step 22, the target base station determines in step 23 whether it is possible to assign the same radio resource as that specified in the handoff request message to the corresponding BTS with regard to one service required by the MSC. If it is not possible to assign the same resource in step 23, the target base station determines in step 24 whether it is possible to assign a radio resource by allowing a change in the Service Configuration Record (SCR). If it is not possible to assign a radio resource by allowing a change in the Service Configuration Record in step 24, the target base station creates a handoff failure message with a Cause value indicating non-assignability of the corresponding radio resource, and then sends the created handoff failure message to the mobile switching center MSC in step 25.
If it is possible to assign a radio resource by allowing a change in the Service Configuration Record in step 24, the target base station creates a handoff request Ack message with a changed IS-2000 Service Configuration Record added thereto, and sends the created handoff request Ack message to the MSC, in step 25-1.
If it is possible to assign the same radio resource in step 23, it means that it is possible to assign the same radio resource as that specified in the IS-2000 Service Configuration Record required by the MSC. Thus, in step 25-2, the target base station creates a handoff request Ack message with the IS-2000 Service Configuration Record removed, and then sends the created handoff request Ack message to the MSC.
If it is not possible to accept the handoff in step 22, the target base station creates a handoff failure message with a Cause value indicating unacceptability of the handoff, and then sends the created handoff failure message to the MSC, in step 25-3.
As described above, the conventional handoff process considers only the case where the mobile communication system provides a single service (not concurrent service). That is, since the conventional mobile communication system provides any one of the voice service and the packet service, it never considers the handoff process performed in a state where the voice and packets services are concurrently provided, i.e., a state where the concurrent service is provided.
Therefore, for the future mobile communication system supporting the concurrent service, it is necessary to consider a plan to perform a concurrent handoff on the two services. If it is not possible to perform a concurrent handoff on the two services, it is necessary to consider a plan to perform a handoff on at least one of the two services.